In sample analysis instrumentation, and especially in separation systems such as liquid chromatography and capillary electrophoresis systems, smaller dimensions generally result in improved performance characteristics and at the same time result in reduced production and analysis costs. Miniaturized separation systems provide more effective system design, result in lower overhead, and enable increased speed of analysis, decreased sample and solvent consumption and the possibility of increased detection efficiency.
Accordingly, several approaches towards miniaturization for liquid phase analysis have developed in the art; the conventional approach using drawn fused-silica capillary, and an evolving approach using silicon micromachining.
In conventional miniaturized technology the instrumentation has not been reduced in size, rather, it is the separation compartment size which has been significantly reduced. As an example, micro-column liquid chromatography (.mu.LC) has been described wherein columns with diameters of 100-200 .mu.m are employed as compared to prior column diameters of around 4.6 mm.
Another approach towards miniaturization has been the use of capillary electrophoresis (CE), which entails a separation technique carried out in capillaries 25-100 .mu.m in diameter. CE has been demonstrated to be useful as a method for the separation of a variety of large and small solutes. J. Chromatog. 218:209 (1981); Analytical Chemistry 53:1298 (1981).
A major drawback of the above approaches to miniaturization involves the chemical activity and chemical instability of silicon dioxide (SiO.sub.2) substrates, such as silica, quartz or glass, which are commonly used in both CE and .mu.LC systems. More particularly, silicon dioxide substrates are characterized as high energy surfaces and strongly adsorb many compounds, most notably bases. The use of silicon dioxide materials in separation systems is further restricted due to the chemical instability of those substrates, as the dissolution of SiO.sub.2 materials increases in basic conditions (at pHs greater than 7.0).
In order to avoid some of the substantial limitations of conventional .mu.LC and CE techniques, and in order to enable even greater reduction in separation system sizes, there has been a trend towards providing planarized systems having capillary separation microstructures. In this regard, production of miniaturized separation systems involving fabrication of microstructures in silicon by micromachining or microlithographic techniques has been described. See, e.g.: Fan et al., Anal Chem. 66(1):177-184 (1994); Manz et al., Adv. in Chrom. 33:1-66 (1993); Harrison et al., Sens. Actuators, B B10(2):107-116 (1993); Manz et al., Trends Anal. Chem. 10(5):144-149 (1991); and Manz et al., Sensors and Actuators B (Chemical) B1(1-6): 249-255 (1990).
The use of micromachining techniques to fabricate separation systems in silicon provides the practical benefit of enabling mass production of such systems. In this regard, a number of established techniques developed by the microelectronics industry involving micromachining of planar materials, such as silicon, exist and provide a useful and well accepted approach to miniaturization. Examples of the use of such micromachining techniques to produce miniaturized separation devices on silicon or borosilicate glass chips can be found in U.S. Pat. No. 5,194,133 to Clark et al., U.S. Pat. No. 5,132,012 to Miura et al., in U.S. Pat. No. 4,908,112 to Pace, and in U.S. Pat. No. 4,891,120 to Sethi et al.
Although silicon micromachining has been useful in the fabrication of miniaturized systems on a single surface, there are significant disadvantages to the use of this approach in creating the analysis device portion of a miniaturized separation system.
Silicon micromachining is not amenable to producing a high degree of alignment between two etched or machined pieces. This has a negative impact on the symmetry and shape of a separation channel formed by micromachining, which in turn may impact separation efficiency. Also, sealing of micromachined silicon surfaces is generally carried out using adhesives which may be prone to attack by separation conditions imposed by liquid phase analyses. Furthermore, under oxidizing conditions, a silica surface is formed on the silicon chip substrate. Thus, silicon micromachining is fundamentally limited by the chemistry of SiO.sub.2. Accordingly, there has remained a need for an improved miniaturized separation system which is able to avoid the inherent shortcomings of conventional miniaturization and silicon micromachining techniques.